The disclosure relates to a hydraulic drive and to a method for controlling a drive.
Hydraulic drives of this type are implemented in the case of mobile working implements, for example compact and mini diggers, in which each wheel or each chain is assigned a hydraulic motor, which hydraulic motors are individually controllable in order to change the movement parameters, for example the direction of travel and the drive speed.
DE 43 25 703 A1 discloses a hydrostatic drive, in which hydraulic motors assigned to each wheel of a wheeled vehicle are hydraulically connected via continuously adjustable directional control valves to a pump assigned to the two hydraulic motors. Said pump is designed as an axial piston pump which is pivotable about zero, wherein the volumetric flow of pressure medium and the direction of flow of the pressure medium to the respectively assigned hydraulic machine is settable by individual control of the continuously adjustable directional control valves.
A drive of this type may be used, for example, in a hydraulic system of a mobile working implement, in which, in addition to the hydrostatic drive, all or some working functions are also supplied via the variable displacement pump. Hydraulic control arrangements of this type are designed as an LUDV system, as described, for example, in DE 10 2006 002 920 A1 belonging to the applicant. In this case, a continuously adjustable directional control valve with an LUDV pressure balance connected downstream is assigned in each case to each working function, for example to differential cylinders for adjusting a boom, a leg, a shovel or a hydraulic motor for adjusting the slewing gear. Said LUDV pressure balance is acted upon by the maximum load pressure of all of the consumers, with the effect of reducing a throttle cross section, and by the pressure downstream of a metering diaphragm, which is formed by the assigned directional control valve, with the effect of increasing a throttle cross section. In the regulating position of each LUDV pressure balance, the pressure in the pressure medium flow path between the assigned metering diaphragm and the respective LUDV pressure balance corresponds to the maximum load pressure which is then throttled via said LUDV pressure balance to the individual load pressure of the particular consumer. Via said LUDV pressure balance, the pressure drop across the metering diaphragms is kept constant irrespective of the load pressure, wherein, in the event of a saturation deficit, the volumetric flow of pressure medium to all of the controlled consumers of a circuit is reduced by the same ratio. In this case, the variable displacement pump is preferably likewise controlled depending on the maximum load pressure of all of the connected consumers such that the pump pressure is at a predetermined pressure differential above said maximum load pressure.
During cornering, the load moment at the hydraulic motor on the outside of the curve rises and correspondingly drops at the hydraulic motor on the inside of the curve. The resulting large pressure difference is compensated for by the LUDV pressure balance of the directional control valve which supplies the drive motor on the inside of the curve. High hydraulic losses occur and the power regulator of the variable displacement pump may reduce the volumetric flow because of the high pressure load on the drive motor on the outside of the curve, which leads to a significant reduction in the cornering speed. A further disadvantage of conventional solutions of this type consists in that, due to manufacturing tolerances, the maximum volumetric flows of pressure medium to the two drive motors cannot be set precisely to the same value, and therefore additional measures have to be taken in order to assist straight line stability.
In the publication “Hydrostatische Antriebe mit Sekunddärregelung [Hydrostatic drives with secondary regulation]”, Der Hydraulik-trainer volume 6; Vogel-Buchverlag, Wurzburg, 1989, a drive for a dipper dredger with secondary regulation is shown, wherein each drive and the additional working functions are supplied with pressure medium via a variable displacement pump arrangement, and each chain of the dipper dredger is assigned two adjustable drive motors. The system is implemented with secondary regulation, in which the rotational speed regulated drive motors are connected to a pressure network and are therefore charged with an impressed pressure. That is to say, in this solution, the rotational speed of the drive motors is regulated in such a manner that said rotational speed is achieved with the impressed network pressure irrespective of the respective load pressure.
A secondary regulation of this type requires a considerable outlay in terms of regulation technology. A further disadvantage of this known solution is that, if the regulating unit breaks down, the drive motors break down, and therefore the mobile working implement is only able still to be moved with great difficulty.
By contrast, it is the object of the disclosure to provide a simply constructed hydrostatic drive which is operable with low hydraulic losses and has improved operational reliability. It is furthermore the object of the disclosure to provide a method for controlling such a hydrostatic drive.